That staple of crime novels - solving a case by identifying the typewriter used to write a ransom note - is being updated for the modern day.
The source of fake cash can now be spotted
US scientists have discovered that every desktop printer has a signature style that it invisibly leaves on all the documents it produces.
They have now found a way to use this to identify individual laser printers.
The work will help track down printers used to make bogus bank notes, fake passports and other important papers.
Before now it was thought that the differences between cheap, mass-produced desktop printers were not significant enough to make individual identification possible.
But a team from Purdue University in Indiana led by Professor Edward Delp has developed techniques that make it possible to trace which printer was used to produce which document.
In 11 out of 12 tests, the team's methods identified which model of desktop laser printer was used to print particular documents.
"We also believe that we will be able to identify not only which model of printer was used but specifically which printer was used," Professor Delp said.
It's not just typewriters that have distinctive patterns
The image processing software developed by Professor Delp's team looks for the "intrinsic signatures" that each printer produces.
Professor Jan Allebach, who helped develop the ID techniques, said the production methods demanded by competition in the desktop printer market meant there was quite a lot of variation in the way different machines printed pages.
"For a company to make printers all behave exactly the same way would require tightening the manufacturing tolerances to the point where each printer would be too expensive for consumers," he said.
The differences emerge in the way that a laser printer lays down ink on the paper and which can be spotted with the Purdue system.
Inkjet is next
Typically, different printers lay down ink in distinct bands that can be spotted by image processing software.
"We extract mathematical features, or measurements, from printed letters, then we use image analysis and pattern-recognition techniques to identify the printer," said Professor Delp.
Desktop printers coupled with scanners have become favourites with forgers as they produce high-quality copies of banknotes and personal documents that can fool a casual glance.
The team is now working to extend its techiques to cover inkjet printers.
The team is also working on ways to manipulate printers so they lay down ink with more easily identifiable signatures.
The researchers will present their detailed findings at the International Conference on Digital Printing Technologies in early November.